Electron beam device



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United States Patent M 3,381,160 ELECTRON BEAM DEVICE Charles L. Andrews, Albany, N.Y., assignor to General Electric Company, a corporation of New York Filed June 29, 1965, Ser. No. 467,855 8 Claims. (Cl. 31514) This invention relates to a device for directing an electron beam to a utilization means without exposing such means to heat or light radiation from an electron source.

A number of devices require the generation of an electron beam for scanning or deflection in a prescribed pattern across a sensitive target. One example is the television type camera tube wherein an electron beam deposits or attempts to deposit electrons upon a light-sensitive target in response to photosensitive currents in the target. The electron flow in the beam then provides output current variations for the tube. The ordinary electron emitting cathode or filament generating an electron beam exhibits certain disadvantages in that such electron source not only emits electrons but also radiates light and heat and is subject to the back bombardment of ions produced within the electron optical system. The radiation deleteriously aifects the sensitivity of a target, particularly in the case of one sensitive in the infrared range, while ion bombardment shortens the life of the electron emitting cathode.

One manner of solving the problem involves separating or turning the electron stream from the radiated heat and light present in the electron source. Thus an electron stream can be deflected at some angle with respect to the source, while heat and light radiation, and to some extent ion back bombardment, continue to follow a straight line path. Prior arrangements have, however, been somewhat complex requiring a plurality of beam forming and controlling electrodes, or a circuitous electron beam path including a number of bafiles around which the electron stream is directed while eliminating the unwanted accompanying radiation. I have discovered a simplified means for providing a small and concentrated electron beam which does not require complex beam forming, deflecting or controlling electrodes. The electron source according to my invention includes only a cathode, and an aperture through which electrons pass, and this aperture is then conveniently focused or imaged at a distance for providing a small, well-defined electron spot. Due to simplicity of construction, the electron source according to the present invention is very efiicient consuming only a small amount of heating power.

According to my invention, a heated cathode has an extended electron emissive surface spaced from an apertured electrode or partition located between the cathode and utilitization means or target. The apertured electrode is connected to a voltage source positive with respect to the cathode for attracting electrons. The aperture is displaced from a direct radiation path between the cathode and target, the device being otherwise free of beam controlling electrodes between cathode and aperture. The cathode is substantially equidistant from the aperture over the extended surface of the cathode and the cathode is spaced from the plane of the aperture in the range of two-thirds to four-thirds the radial distance of the cathode surface from the axis of the aperture. With these proportions observed it is found a large percentage of the electron beam from the cathode passes through the aperture without the use of further electrodes.

In a preferred embodiment of the present invention, the cathode takes the form of a toroid having its inside diameter coated with electron emissive material. A conducting partition or grid electrode between the cathode and the beam utilization target has a central circular aperture whose axis is common with that of the toroidal cath- 3,381,160 Patented Apr. 30, 1968 ICC ode. Again, the cathode is spaced from the plane of the aperture, that is from the conducting partition, in the range of two-thirds to four-thirds the inside radius of the toroidal cathode. A voltage source connected to the conducting partition causes electrons emitted from the inside diameter of the toroidal cathode to curve towards the axis of the aperture with a substantial portion thereof passing through the aperture. For best efliciency, the aperture should have a diameter equal to a substantial fraction of the vertical width of the electron emissive surface inside the toroidal cathode. If the aperture is made at least as large as this width in diameter, or more of electrons are concentrated in the aperture, and the electron density at the aperture is at least three times the electron density at the cathodes electron emissive surface.

Thus, the device according to the present invention provides the advantage of radiation shielding of a target from unwanted heat and light radiation while providing a substantial concentration of electrons forming an electron beam. This result is accomplished without complex electrode structures and therefore the electron source can be made quite small and efiicient requiring a minimum of heating power.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with additional advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is a vertical cross-section of an electron source in accordance with the present invention,

FIG. 2 is a section of the FIG. 1 electron source taken at 2-2,

FIG. 3 is a schematic representation of the source illustrating operation thereof,

FIG. 4 is a plot of equipotential lines and electron paths in the device according to the present invention, and

FIG. 5 is a cross-section of a complete electron beam device in accordance with the present invention.

Referring to FIGS. 1 and 2, an electron source in accordance with an embodiment of the present invention includes a toroidal or annular metal cathode 1, the inside wall of which is provided with a surface 2 of electron emissive material conveniently in the initial form of a barium-strontium carbonate. The cathode is centrally disposed in a cylindrical ceramic enclosure 3 having a first flat end partition 4 acting as a metal electrode, and a second flat end partition 5, which is also metallic. End partition 5 may be connected to the cathode or grounded for shielding purposes.

The partition 4 includes a central aperture 6 having a common axis with toroidal cathode 1. Thus the emissive surface 2 is spaced at a substantially constant distance from aperture 6. The diameter of aperture 6 should be at least equal to a substantial fraction of the vertical width, W, that is the axial extent of electron emissive surface 2,

and W is preferably constant around the inside of the 1 toroid. The diameter of aperture 6 preferably at least equals W. The spacing, S, of the cathode behind partition 4, or behind the plane of aperture 6, is in the range of twothirds to four-thirds the radius, R, of the toroidal cathode. In other words, this spacing is in the range between onethird and two-thirds the inside diameter of the cathode, and preferably equals the radius, R, for reasons which will hereinafter become more apparent.

In the illustrated embodiment, the cathode 1 is supported within ceramic enclosure 3 by a Nichrome spring 7, having three equal sides and central slots for receiving cathode 1. The toroidal cathode in the embodiment is a thin walled cylinder of tungsten with the ends ground flat. An annular groove is provided around the outside wall into which is bonded a helical filament of the type used in lamp bulbs, and in this case having 240 turns per inch. This filament 8 extends substantially completely around the grooved periphery of the cathode being provided with end conductors 9 and for connection to a suitable source of heating power. The filament 8 is bonded to the cathode in heat conducting, but electrically insulating relation within ceramic material 11. An effective method for bonding the filament to the cathode in this manner is set forth in the copending application of August I. Kling, Ser. No. 418,591, filed Dec. 7, 1964, entitled Heated Cathode and Method of Manufacture, and assigned to the assignee of the present application. If desired, the heating filament may be somewhat larger than the emitting portion of the cathode in which case the outside of the cathode cylinder can be somewhat enlarged or extended in comparison to the height, W, of the emitting surface.

In operation, the electron source in accordance with the present invention provides a substantially axial beam of electrons through aperture 6, directed towards an electron beam utilization means disposed along the axis of aperture 6. While the electron beam is directed along the axis, partition 4 prevents undesired radiation of heat and light from reaching electron utilization means on this axis, because no direct or straight-line path will exist between the cathode and such utilization means. In operation of the source, a supply of voltage, V is connected to partition 4 with partition 4 acting as a grid. This voltage is positive with respect to the cathode and attracts electrons emitted from surface 2 towards partition 4. A sheet of electrons equal in width to W is attracted and curves towards partition 4. It is found that if spacing, S, is in the range between two-thirds and four-thirds the radius of the cathode, a large proportion of the electron beam passes through central aperture 6, as illustrated in FIG. 3, without requiring complex beam deflection means. The beam at the aperture is approximately equal in width to W, the width of the electron emissive material, and therefore if aperture 6 is equal to a substantial fraction of W, then a substantial fraction of the electron emission will pass through aperture 6. Of course, the size of the aperture determines the actual proportion of the beam width, W, passing therethrough. The aperture 6 is preferably circular and preferably has a diameter which is at least equal to W, the width or axial extent of the electron emissive material and in that case 90% of the electrons pass through the aperture.

Both the width W of the electron emissive strip and the diameter of aperture 6 may be quite small supplying a very small concentrated source of electrons, of course without accompanying heat and light radiation. The diameter of aperture 6 in embodiments constructed was in the range of 20 mils to 1 or 2 mils.

The current density through the aperture is approximately 3 or 4 times the current density from the cathode emitting surface 2, because the electron emission from the entire cathode inside diameter is concentrated or gathered through aperture 6. In addition to the advantages of elimination of unwanted radiation and increased current density, back ion bombardment of the cathode is also eliminated.

It is noted that focusing of the electron stream through aperture 6 is accomplished without beam forming, defleeting or controlling electrodes between the cathode and aperture 6. With the dimension ranges given, the apertured grid partition, 4, is sufiicient alone to bring about the desired concentration. Thus the electron source in accordance with the present invention is simple in construction and can be made much smaller than non-radiating sources heretofore proposed. Because of the electron beam concentration accomplished and with small losses because of fewer electrodes, heating power is minimized.

FIG. 4 represents a plot of electron paths 12 from the cathode superimposed upon equipotential lines of the electric field between the cathode and partition 4. The partition 4 voltage was in this case volts, while the cathode and partition 5 voltage was zero. The cathode had a diameter of 0.25 inch located at its own radius away from partition 4. The manner in which the electron paths cross the equipotential lines and become somewhat more concentrated as they approach the location of an aperture, within an approximate cathode-aperture spacing equal to the cathodes radius, may be observed from the FIG. 4 plot. Focusing is dependent upon a positive voltage upon partition 4, but is not highly sensitive to its value. For this cathode, voltages between 90 and volts were satisfactory.

Returning to FIG. 3, depicting the complete electron beam, this beam, 19, after passing through aperture 6, is somewhat conical in shape and is composed of the sheet of electrons surrounding a hollow center. A fluorescent screen 13, intersecting the beam, will display a well-defined circle of narrow outline where the electron beam strikes the fluorescent material. The narrowness of the outline is determined from the small diameter of aperture 6 and the correspondingly small width, W, of electron emissive material 2 on cathode 1. This type of electron beam has an additional advantage in that it may be focused to a small spot, approximating the dimension W in diameter, using a simple electron lens without encountering aberration problems.

While the cathode preferably forms a complete annulus or toroid, the invention in its broader aspects contemplates a cathode approaching a portion of a toroid, that is one having an extended surface substantially equidistant from the aperture 6. Again, the electron emissive material coated surface desirably extends substantially axially in a direction parallel to the axis of the aperture, with a width comparable to the aperture diameter.

FIG. 5 illustrates a complete electron beam device in accordance with the present invention, including an envelope 14 having a cathode, 1, at one end thereof and a utilization means in the form of a sensitive target 15 at the other end thereof. Target 15 may comprise, for example, a semiconductor target sensitive to infrared radiation and the like, and from which it is desired to exclude heat and light radiated from cathode 1. Accordingly, the electron source of the present invention includes apertured partition 4 whose aperture is displaced from the direct straight-line radiation path between the cathode 1 and target 15. The conical electron beam passing through aperture 6 is focused, employing focusing electrodes 16, 17 and 18, to a small diameter spot where the beam 19 approaches target 15. The electron focusing electrodes 16, 17 and 18 comprise an electron lens acting to image the aperture 6 upon target 15, but because of the hollow cross-section of the electron beam this lens need not correct for aberration effects. It is again noted no beam deflecting or controlling electrodes are interposed between cathode 1 and aperture 6 making it possible for the distance therebetween to be relatively small and therefore for the electron source to be quite small and efiicient.

While I have shown and described a preferred embodiment of my invention, is will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron beam device for providing electrons to a utilization target without exposing the utilization target to heat or light radiation from a heated electron source comprising an envelope with a cathode at one end thereof and an electron utilization target at the other; a conducting partition bteween said cathode and electron utilization target provided with an aperture displaced from a direct radiation path between said cathode and said electron utilization target, the device being otherwise free of beam controlling electrodes between said cathode and said aperture; said cathode having an extended electron emissive material coated surface displaced from the axis of said aperture by a substantially constant radial distance; means for heating said cathode to provide thermionic emission of electrons therefrom; and voltage supply means connected to said partition causing emitted electrons to curve towards said partition and through said aperture; said cathode surface having a spacing from the plane of said aperture substantially in the range of two-thirds to fourthirds the radial distance of said cathode from the axis of said aperture.

2. An electron beam device for providing electrons to a utilization target without exposing the utilization target to heat or light radiation from a heated electron source comprising an envelope with a cathode at one end thereof and an electron utilization target at the other; a conducting partition between said cathode and electron utilization target provided with an aperture displaced from a direct radiation path between said cathode and said electron utilization target, the device being otherwise free of beam controlling electrodes between said cathode and said aperture; said cathode having an extended electron emissive material coated surface extending substantially in a direction parallel to the axis of said aperture but displaced therefrom; and having an edge substantially equidistant from said aperture; means for heating said cathode to provide thermionic emission of electrons therefrom; and voltage supply means connected to said partition causing emitted electrons to curve towards said partition and through said aperture, wherein the size of said aperture is large enough to pass a large proportion of a beam of electrons therethrough equal in diameter to the axial extent of the electron emitting surface of said cathode; said cathode having a spacing from the plane of said aperture in the range of between two-thirds and four-thirds the radial distance of said cathode from the axis of said aperture.

3. An electron beam device for providing electrons to a utilization means without exposing said utilization means to direct radiation from a heated electron source comprising an envelope with a cathode at one end thereof and an electron utilization means at the other; a planar conducting grid between said cathode and said electron utilization means provided with a circular aperture displaced from a direct radiation path between said cathode and said utilization means, the device being otherwise free of beam controlling electrodes between said cathode and said aperture; said cathode having an annular electron emissive material coated surface extending substantially in a direction parallel to the axis of said aperture with the axis of the annulus substantially coinciding with the axis of the aperture so that the edge of said electron emissive surface is substantially equidistant from the edge of said aperture; means for heating said cathode to provide thermionic emission of electrons therefrom; voltage supply means higher in potential than said cathode causing emitted electrons to curve towards said grid and the axis of said aperture passing through said aperture; and an electron lens between said grid and said utilization means for focusing said circular aperture upon said utilization means causing a small diameter beam of electron to approach said utilization means, wherein the diameter of said aperture is large enough to pass a substantial fraction of a beam of electrons therethrough, the beam being equal in diameter to the axial extent of the electron emitting surface of said cathode; said cathode having a spacing from the plane of said grid in the range of two-thirds to four-thirds the distance of said cathode from the axis of said aperture.

4. An electron source including a first conducting planar partition provided with a circular aperture, a toroidal cathode spaced behind said partition having a common axis with said circular aperture and provided with an electron emissive inner surface having a substantially constant axial extent in a direction parallel to the axis of said aperture, said source being free of beam controlling electrodes between said cathode and said aperture, a heater element in heat conducting relation to said annular cathode to excite thermionic emission of electrons therefrom, voltage supply means connected to said partition higher in potential than said cathode for causing emitted electrons to curve towards the axis of said aperture and pass through said aperture, and a planar second partition behind said toroidal cathode having a potential substantially equal to said cathode, said toroidal cathode having a spacing from said first partition in the range of one-third to two-thirds the diameter of said toroidal cathode.

5. An electron source including a planar partition provided with a circular aperture, a toroidal cathode spaced behind said partition and having a common axis with said circular aperture and provided with an electron emissive surface having a substantially constant axial extent in a direction substantially parallel to the axis of said aperture, said source being free of beam controlling electrodes between said cathode and said aperture, a heater element in heat conducting relation to said annular cathode to excite thermionic emission of electrons therefrom, and voltage supply means connected to said partition causing emitted electrons to curve towards the axis of said aperture and pass through said aperture, said toroidal said cathode having a spacing from said partition in the range of one-third to two-thirds the diameter of said toroidal cathode, and the diameter of said aperture being at least as large as the axial extent of said electron emissive surface.

6. An electron source including a conducting planar partition provided with a circular aperture, a toroidal cathode space behind said partition having -a common axis with said circular aperture and provided with an electron emissive inner surface, said source being free of beam controlling electrodes between said cathode and said aperture, a heater element in heat conducting relation to said annular cathode to excite thermionic emission of electrons therefrom, and voltage supply means connected to said partition causing emitted electrons to curve towards the axis of said aperture and pass through said aperture, said toroidal cathode having a spacing from said partition in the range of one-third to twothirds the diameter of said toroidal cathode with the diameter of said aperture being at least equal to a substantial fraction of the axial extent of said electron emissive surface for passing a substantial beam of electrons therethrough.

7. An electron source including a conductive partition provided with an aperture, a cathode disposed behind said partition having a shape of a cylinder whose axis substantially coincides with that of the aperture, means spacing said cylinder from said partition with the space between said cylinder and said partition being free of other electrodes, said cylinder being provided with an electron emissive coating around the inside of said cylinder and with said coating having a substantially constant spacing from said aperture, heating means in heat conductive relation to said cylinder to excite thermionic emission of electrons from the inner electron emissive surface of said cylinder, and voltage supply means connected to said partition of a value for causing emitted electrons to curve towards the axis of said aperture with a major portion concentrated at said aperture and with a substantial portion passing through said aperture, the spacing of said electron emissive surface relative to the plane of said aperture being in the range of one-third to twothirds the diameter of said cylinder.

8. An electron source including a conductive partition provided with a circular aperture, a cathode disposed behind said partition having a shape of a cylinder whose axis substantially coincides with that of the aperture, means spacing said cylinder from said partition With the space between said cylinder and said partition being free of other electrodes, said cylinder being provided on its inner surface with an electron emissive coating of substantially constant axial extent around the inside of said cylinder and with the edge of said coating nearest said partition having a substantially constant spacing from said aperture, heating means in heat conductive relation to said cylinder to excite thermionic emission of electrons from the inner electron emissive surface of said cylinder, and voltage supply means connected to said partition of a value for causing emitted electrons to curve towards the axis of said aperture with a major portion thereof passing through said aperture, said aperture References Cited UNITED STATES PATENTS 2,038,341 4/1936 Bruche 313193 2,362,937 11/1944 Shepherd 313339 X 2,452,044 10/1948 Fox 313339 X RODNEY D. BENNETT, Primary Examiner.

M. F. HUBLER, Assistant Examiner. 

3. AN ELECTRON BEAM DEVICE FOR PROVIDING ELECTRONS TO A UTILIZATION MEANS WITHOUT EXPOSING SAID UTILIZATION MEANS TO DIRECT RADIATION FROM A HEATED ELECTRON SOURCE COMPRISING AN ENVELOPE WITH A CATHODE AT ONE END THEREOF AND AN ELECTRON UTILIZATION MEANS AT THE OTHER; A PLANAR CONDUCTING GRID BETWEEN SAID CATHODE AND SAID ELECTRON UTILIZATION MEANS PROVIDED WITH A CIRCULAR APERTURE DISPLACED FROM A DIRECT RADIATION PATH BETWEEN SAID CATHODE AND SAID UTILIZATION MEANS, THE DEVICE BEING OTHERWISE FREE OF BEAM CONTROLLING ELECTRODES BETWEEN SAID CATHODE AND SAID APERTURE; SAID CATHODE HAVING AN ANNULAR ELECTRON EMISSIVE MATERIAL COATED SURFACE EXTENDING SUBSTANTIALLY IN A DIRECTION PARALLEL TO THE AXIS OF SAID APERTURE WITH THE AXIS OF THE ANNULUS SUBSTANTIALLY COINCIDING WITH THE AXIS OF THE ANNULUS SUBSTANTIALLY COINCIDING ELECTRON EMISSIVE SURFACE IS SUBSTANTIALLY EQUIDISTANT FROM THE EDGE OF SAID APERTURE; MEANS FOR HEATING SAID CATHODE TO PROVIDE THERMOINIC EMISSION OF ELECTRONS THEREFROM; VOLTAGE SUPPLY MEANS HIGHER IN POTENTIAL THAN SAID CATHODE CAUSING EMITTED ELECTRONS TO CURVE TOWARDS SAID GRID AND THE AXIS OF SAID APERTURE PASSING THROUGH SAID APERTURE; AND AN ELECTRON LENS BETWEEN SAID GRID AND SAID UTILIZATION MEANS FOR FOCUSING SAID CIRCULAR APERTURE UPON SAID UTILIZATION MEANS CAUSING A SMALL DIAMETER BEAM OF ELECTRON TO APPROACH SAID UTILIZATION MEANS, WHEREIN THE DIAMETER OF SAID APERTURE IS LARGE ENOUGH TO PASS A SUBSTANTIAL FRACTION OF A BEAM OF ELECTRONS THERETHROUGH, THE BEAM BEING EQUAL IN DIAMETER TO THE AXIAL EXTENT OF THE ELECTRON EMITTING SURFACE OF SAID CATHODE; SAID CATHODE HAVING A SPACING FROM THE PLANE OF SAID GRID IN THE RANGE OF TWO-THIRDS TO FOUR-THIRDS THE DISTANCE OF SAID CATHODE FROM THE AXIS OF SAID APERTURE. 